Loudspeakers generally comprise a frame, a motor structure, a diaphragm, a lower suspension or spider and a surround. In one common type of speaker, the motor structure includes a permanent magnet mounted between a top plate and a back plate, a pole piece centrally mounted on the back plate and a voice coil axially movable with respect to the pole piece. The voice coil includes a hollow, cylindrical-shaped former having an outer surface which receives a winding of wire.
One end of the diaphragm is connected to the surround or upper suspension, which, in turn, is mounted to the upper end of the frame. The lower suspension or spider is connected at one end to a seat formed in the frame at a point between its upper and lower ends. The free ends of the diaphragm and spider are mounted to the voice coil and support it within the air gap between the pole piece and top plate of the motor structure, with the former of the voice coil concentrically disposed about the pole piece. In some speaker designs, a dust cap is mounted to the diaphragm in position to overlie the voice coil and pole piece to protect them from contaminants. This forms a dust cap cavity between the dust cap, diaphragm and the voice coil and pole piece. In alternative designs, the upper end of the voice coil is connected directly to the diaphragm, thus eliminating the need for a dust cap but nevertheless forming an internal or dust cap cavity in the area directly above the voice coil and pole piece.
In the course of operation of a speaker of the type described above, electrical energy is supplied to the voice coil causing it to axially move relative to the pole piece and within the air gap formed between the top plate and pole piece. The diaphragm, spider and surround, move with the excursion of the voice coil. A pervasive problem associated with speaker operation involves the build up of heat produced by the voice coil and radiated to surrounding surfaces, particularly the top plate. Both the voice coil and top plate become quite hot during speaker operation which can reduce the power handling of the speaker, and increase power compression, i.e. a reduction in acoustic output due to temperature-related voice coil resistance.
A variety of designs have been employed in the prior art to address the problems associated with heat build up in speakers. One approach has been to create a flow of cooling air in thermal communication with the voice coil, such as disclosed, for example, in U.S. Pat. No. 5,042,072 to Button, U.S. Pat. No. 5,357,586 to Nordschow et al. and U.S. Pat. No. 5,426,707 to Wijnker. Speaker designs of this type generally include a pole piece formed with passages which provide a flow path for the transfer of cooling air from outside of the speaker into and out of the dust cap cavity described above. An air flow through these passages is created in response to movement of the diaphragm with the excursion of the voice coil. When the diaphragm moves in one direction, air is drawn from outside of the speaker, through vent openings in the back plate, along the passages in or along the pole piece, and then into the dust cap cavity. Movement of the diaphragm in the opposite direction creates a flow out of the cavity along the reverse flow path.
In the Button U.S. Pat. No. 5,042,072, the pole piece of the motor is formed with a series of circumferentially spaced, longitudinally extending grooves or channels. Each channel extends radially inwardly from the outer surface of the pole piece toward its center, and from the top end of the pole piece to its bottom end including in the area of the air gap between the pole piece and top plate. The purpose of the radial channels in the pole piece is to direct a flow of air along the voice coil as the air passes in and out of the dust cap cavity. Although it is contemplated that at least some of the air flow contacts the voice coil in this design, because the radial channels in the pole piece are oriented parallel to the voice coil along the longitudinal axis of the pole piece a limited amount of the cooling air actually impinges directly against the voice coil. Additionally, the formation of a number of radial channels in the pole piece reduces its mass in the area of the air gap with the top plate. This increases the reluctance of the magnetic path between the pole piece and top plate resulting in a decrease in motor strength which can adversely impact the acoustic performance of the speaker.
U.S. Pat. No. 5,357,586 to Nordschow employs a pole piece including a central throughbore forming an annular wall defining a hollow interior. An aerodynamically-shaped insert is mounted within the central bore of the pole piece by a series of fins or spacers, thus forming longitudinally extending channels between the insert and the wall. Additionally, the wall of the pole piece is formed with a number of transverse bores extending between its outer surface and the central bore. In response to movement of the voice coil and diaphragm in one direction, air from outside of the speaker is drawn into the central bore of the pole piece, through its transverse bores, along the exterior surface of the pole piece into the air gap between the pole piece and top plate, and then through bores formed in the voice coil into the dust cap cavity. Movement of the diaphragm in the reverse direction causes a flow of air out of the cavity through the voice coil bores, and then predominantly through the central bore of the pole piece along the channels formed by the fins of the aerodynamically-shaped insert.
Although the intention in the '586 patent is to cool the voice coil, it is unlikely that any effective cooling would occur with this design. The air gap between the pole piece and top plate is exceedingly small, considering that particularly the voice coil is located therein, and no appreciable amount of air flow can be created through the air gap without using a design such as described in the '072 Button patent wherein longitudinal channels are formed in the pole piece to provide a flow pat h between the pole piece and the top plate. The '586 patent does not include a pole piece with longitudinal channels along its exterior surface, but instead attempts to force a flow of air from the transverse bores in the pole piece through the air gap, and, hence, along the outer surface of the voice coil. Additionally, the flow of air in the reverse direction noted above is for venting purposes only and does not result in the movement of cooling air along or adjacent to the wire winding of the voice coil.
The '707 patent to Wijnker is similar to Nordschow et al. in that it includes in one embodiment a pole piece formed with a central bore and a number of transverse bores extending through the wall of the pole piece. The transverse bores in Wijnker are employed to create a flow of air from outside of the speaker, into the central bore of the pole piece and then out the transverse bores to discharge ports formed in the back plate of the speaker. No cooling air passes from the transverse bores, along the voice coil and into and out of the dust cap cavity. Alternative embodiments of Wijnker disclose a flow path into and out of the dust cap cavity, but employ a pole piece formed with a throughbore and no transverse bores and wherein an attempt is made, as in Nordschow et al., to force air to flow within the air gap between the top plate and pole piece.